GASMAX ECx Instruction Manual
Revision 1.1
10
3.4 System
Design Specifications
Power Supply:
Integral non-rechargeable 3.6 volt 19AH lithium D cell battery. Replacement part
# 10-0293.
Power Consumption:
<2mA during “sleep” mode, 40mA during “receive beacon” mode, up to 1
amp during 1 watt “transmit” mode. Transmit power may be set from 10mW to 1 watt (see
Section 6-7)
Transmit (TX) Power:
30dBm at highest 1W power setting. Transmit power may be set from
10mW to 1 watt (see Section 6-7)
Receive (RX) Sensitivity:
-100 dBm
Radio Frequency:
Hopping occurs between 902 – 928MHZ.
Memory:
Non-volatile E
2
memory retains configuration values in the event of power outages.
3.5
Antenna Transmission Range
The distance radio signals can travel is dependent upon several factors including antenna design,
transmitter power and free space losses.
In order for a wireless link to work, the available system
operating margin
(TX power - RX Sensi Antenna gains)
must exceed the free space loss
and all other losses in the system. For best RF line-of-site, the combined height of both antennas
must exceed the Fresnel zone diameter (see below).
Dist. between ant's
Fresnel zone diameter Freespace loss (dB)
1000 ft (300 m)
16 ft (4.9 m)
81
1 Mile (1.6 km)
32 ft (9.7 m)
96
5 miles (8 km)
68 ft (20.7 m)
110
10 miles (16 km)
95 ft (29 m)
116
Example:
The RF900 radio modem has the following parameters:
•
Maximum RF900 TX power setting = 30 dBm (1 Watt)
•
RF900 RX sensitivity = -100 dBm (this is a constant)
•
Antenna gain (standard equipped dipole) = 2.1dBi x 2 = 4.2dBi
So the system operating margin at ten miles is 30 + 2.1 -116 + 2.1 + 100 = 18.2 dBm. This
margin should be enough to transmit 10 miles if free space was the only loss in the system. For
this to be the case, however, the antennas must be mounted with a combined height greater than
95ft above all obstructions. In practice however, there are many losses in the system besides just
free space and it is recommended there be at least 20dB to 40dB system operating margin.
RF “Rules of Thumb”
•
Doubling the range with good RF “Line of Sight” (LOS) increases the free space loss by
at least 6 dB.
•
Doubling the range without good RF LOS increases the free space loss by at least 12 dB.